Endotoxins, or lipopolysaccharides (LPS), the predominant structural component of the outer membrane of Gram-negative bacteria,1-3 play a pivotal role in septic shock, a syndrome of systemic toxicity which occurs frequently when the body's defense mechanisms are compromised or overwhelmed, or as a consequence of antibiotic chemotherapy of serious systemic infections (Gram-negative sepsis).4-7 Referred to as “blood poisoning” in lay terminology, Gram-negative sepsis is the thirteenth leading cause of overall mortality8 and the number one cause of deaths in the intensive care unit,9 accounting for more than 200,000 fatalities in the US annually.10 Despite tremendous strides in antimicrobial chemotherapy, the incidence of sepsis has risen almost three-fold from 1979 through 200011 and sepsis-associated mortality has essentially remained unchanged at about 45%12, both calling to attention the fact that aggressive antimicrobial therapy alone is insufficient in preventing mortality in patients with serious illnesses, and emphasizing an urgent, unmet need to develop therapeutic options specifically targeting the pathophysiology of sepsis.
The presence of LPS in the systemic circulation causes a widespread activation of the innate immune response13;14 leading to the uncontrolled production of numerous inflammatory mediators, including tumor necrosis factor-α (TNF-α), interleukin-1 β (IL-1β), and interleukin-6 (IL-6), primarily by cells of the monocyte/macrophage lineage,15;16 as well as others, such as nitric oxide produced by the endothelial cell,17;18 which, in concert, act to cause a frequently fatal systemic inflammatory response,19 termed ‘septic shock’. The toxic moiety of LPS is its structurally conserved glycolipid component called Lipid A,20 which is composed of a hydrophilic, bis-phosphorylated diglucosamine backbone, and a hydrophobic domain of 6 (E. coli) or 7 (Salmonella) acyl chains20 (FIG. 1). The pharmacophore necessary for the neutralization of lipid A21 by small molecules requires two protonatable positive charges separated by a distance of ˜14 Å, enabling ionic H-bonds between the cationic groups and the lipid A phosphates; in addition, appropriately positioned pendant hydrophobic functionalities are required to further stabilize the resultant complexes via hydrophobic interactions with the polyacyl domain of lipid A (for a recent review, see Ref.22). These structural requisites were first identified in certain members of a novel class of compounds, the lipopolyamines, which were originally developed, and are currently being used as DNA transfection (lipofection) reagents.23-26 In a detailed study of the effect of the hydrocarbon chain length in a homologous series of acylhomospermines, it was shown that C16 is the ideal lipophilic substituent, corresponding to maximal affinity, optimal aqueous solubility (and bioavailability), and neutralization potency.27 